From Scientific American:
Up to 20 percent of women who develop lung cancer have never smoked, U.S. researchers found in a study that suggests secondhand smoke may be to blame. A survey of a million people in the United States and Sweden shows that just 8 percent of men who get lung cancer are nonsmokers.
She said it is not clear why women may be more likely to get lung cancer even if they have never smoked. “There is a lot of controversy over whether women are more susceptible to smoking at all, whether direct or secondhand smoke,” Wakelee said in a telephone interview.
Among women who never smoked, the lung cancer incidence rate ranged from 14.4 per 100,000 women per year to 20.8 cases per 100,000. In men, it ranged from 4.8 to 13.7 per 100,000. Rates were about 10 to 30 times higher in smokers. This would translate to about 20 percent of female lung cancer patients having been nonsmokers and 8 percent of males.
Smoking is by far the leading cause of lung cancer, but radon, asbestos, chromium and arsenic are also associated with lung cancer.